The present invention relates generally to the field of optics, and more specifically to a combined optical light pipe and protective enclosure.
Total internal reflection is an optical phenomenon that occurs when light rays are reflected at the boundary between two transparent media, each having a different index of refraction. Light rays passing through a first transparent medium of relatively high refractive index (optically dense) and arriving at the interface with a second transparent medium of lower refractive index (optically rarer) may be completely reflected within the optically dense medium. Total internal reflection depends on the difference between the refractive indices of the two media and the angle of incidence of the light ray at the interface. The critical angle for total internal reflection is defined as the minimum angle at which a light ray striking the surface of an optically rarer medium will be reflected instead of refracted. Although the reflection is total, the light ray actually penetrates a small distance into the rarer medium.
Total internal reflection can be mathematically expressed by Snell""s Law. Snell""s Law may be expressed as:
sin xc3xa1/sin xc3xa2=n2/n1 
where n1 is the index of refraction of the optically dense medium, n2 is the index of refraction of the optically rare medium, xc3xa1 is the angle traversed by a beam of light travelling through the first optically dense medium, incident upon the interface and a line erected normal to the point of incidence, and xc3xa2 is the angle between the beam of light refracted into the second optically rare medium and a line erected normal to the point of entry into that medium. For the beam to be totally internally reflected into the first optically dense medium, xc3xa2 must be at least 90 degrees. If xc3xa2=90 degrees, the beam is reflected parallel to the interface between the two media. If xc3xa2 is greater than 90 degrees, the beam is totally reflected back through the first optically dense medium. The critical angle for total internal reflection, xc3xa1CRIT, is thus:
sin xc3xa1CRIT/sin 90xc2x0 greater than n2/n1 
or
xc3xa1CRIT greater than sinxe2x88x921n2/n1 
A light pipe is a kind of optical waveguide that utilizes the phenomena of total internal reflection to direct light from one point to another. Light pipes are convenient for bending light, directing light around corners, splitting a light beam for delivery to a plurality of destinations, precisely directing light from a specific source to a specific destination, transmitting light signals through environments otherwise hostile to light transmission, insulating light beams from outside interference or scattering, and like applications. Since light pipes operate by the principle of total internal reflection, it is important that the interfacial surface of the light pipe be kept clean. For most light pipes, the optically rarer medium is air. If the optically denser light pipe surface becomes contaminated with a contaminant optically less rare than air, the n2/n1 value increases and the critical angle xc3xa1CRIT accordingly increases, allowing for light to escape or xe2x80x9cleakxe2x80x9d from the light pipe, thereby reducing its efficiency and utility. In extreme cases, optical leakage can result in the catastrophic failure of the light pipe. Therefore, it is necessary to protect the light pipe from such contaminants.
In some applications, there is scant room for the placement of the light pipe. Where miniaturization is necessary, it becomes difficult to provide room for the light pipe itself, and provision of a protective structure is often impossible or prohibitively difficult. One protection technique that is commonly used is to provide an optically rare coating over the optically dense light pipe. However, the interface between the environment (typically air) and the optically rare coating may still be degraded by contaminants such that some leakage still occurs. Coatings are typically thin and more fragile than the underlying light pipe, and may be scratched, abraded, or chemically attacked in their environments. Such coating degradation further decreases the efficiency of the light pipe. Moreover, the addition of a coating increases the cost of the light pipe system. There is therefore a need for a light pipe design that more efficiently utilizes available space without requiring extraneous contaminant protection structures or protective coatings. The present invention addresses this need.
One embodiment of the present invention is a housing having an optically transparent internal volume of a first refractive index with an optical entry point defined on the surface of the optically transparent volume and an optical transit point defined within the optically transparent volume, and a first pocket formed in the optically transparent volume. The first pocket defines a first three-dimensional negative object within the optically transparent volume having a second refractive index substantially less than the first refractive index. The first pocket is substantially encased by the optically transparent volume and is shaped to reflect a beam of light passing through the optical entry point and incident upon the first pocket to the optical transit point.
One object of the present invention is to provide an improved light pipe. Related objects and advantages of the present invention will be apparent from the following description.